The so called sub-field method of the prior art, as described in Japanese Patent Laid-Open Publication No. H04-195087, is used in display devices having a binary memory effect (such as PDP) for displaying luminous half tones. FIGS. 30A and 30B show an example of this method. The image display device writes down control data for turning luminescence on and off in advance for all picture elements of a display screen, and then illuminates all the picture elements at once according to the control data. This method enables the image display device to display television pictures having 256 gradations of luminous tone of eight bit coding. An example of this method is described below.
An example of the prior art in which one field of image is constituted by eight sub-fields of binary image, as shown in FIG. 30A, is now described. Each of the sub-fields has a luminant period (a period in which any sub-fields illuminate during an ON state) and a nonluminant period, and shaded portions are the luminant periods. A time length of the luminant period or a number of pulses illuminated during the ruminant period corresponds to the weight given according to the luminous level, although the nonluminant periods are nearly equal throughout every sub-field. Each sub-field is assigned with a sub-field number, and a different weight is given on each sub-field having a sub-field number.
The sub-field method obtains luminous gradations by varying the time length of luminous level or the number of luminant pulses within a time period which is a period for one field (a lapse of time) in which an afterimage of human vision is available. Humans perceive a luminous level of each picture element as an integrated sum of the illuminated time or as a cumulative number of luminant pulses with respect to individual picture elements in each sub-field of one field.
In the example of FIGS. 30A and 30B, each sub-field is given with a weight (hereinafter called "luminous level") corresponding to the luminous level of 1, 2, 4, 8, 16, 32, 64 and 128 respectively according to the binary notation. For example, a sub-field having a sub-field number of "1" (hereinafter called "sub-field 1") illuminates once in order to produce a luminous level of "1", and a sub-field of "sub-field 8" illuminates 128 times in order to produce a luminous level of "128".
FIG. 30B shows the sub-fields to be illuminated so as to display the required luminous gradations. The sub-fields and a weight given to each of the sub-field numbers are shown on the abscissa, and the luminous gradations to be displayed are shown on the ordinate. Sections that are marked with "ON" in the diagram indicate the sub-fields to be illuminated for displaying the luminous gradations on the ordinate.
More specifically, the sub-field 1 is illuminated for displaying the luminous gradation 1. Likewise, it illustrates the sub-field 2 for displaying the luminous gradation 2, the sub-fields 1 and 2 for displaying the luminous gradation 3, the sub-field 3 for displaying the luminous gradation 4, the sub-fields 1 and 3 for displaying the luminous gradation 5, the sub-fields 2 and 3 for displaying the luminous gradation 6, the sub-fields 1, 2 and 3 for displaying the luminous gradation 7, the sub-fields 4 in combination with those of the luminous gradations 0 to 7 for displaying the luminous gradation 8 through 15, the sub-fields 5 in combination with those of the luminous gradations 0 to 15 for displaying the luminous gradation 16 through 31, the sub-fields 6 in combination with those of the luminous gradations 0 to 32 for displaying the luminous gradation 32 through 63, the sub-fields 7 in combination with those of the luminous gradations 0 to 64 for displaying the luminous gradation 64 through 127, and the sub-fields 8 in combination with those of the luminous gradations 0 to 128 for displaying the luminous gradation 128 through 255, respectively.
All individual picture elements of the PDP display the half tones luminous level by combining the sub-fields to be illuminated in this manner. To obtain a luminous gradation of "173" for example, sub-fields to be illuminated are the sub-field 8 having a weighting of "128", the sub-field 6 having a weighting of "32", the sub-field 4 having a weighting of "8", the sub-field 3 having a weighting of "4" and the sub-field 1 having a weighting of "1". In this way, the PDP illuminates in response to the weighting (or illuminates a number of times according to the weighting), and the resulting luminous level (that humans perceive) is in proportion to an integrated sum of the illumminated time.
Using this method for displaying luminous half tones when showing still images, a desired half tones is realized without giving a disorderly impression (or any other problems) of picture quality. This is because humans perceive a luminous level of each picture element by properly adding the weight given to each of the sub-fields within an elapsing time period for one field because humans eyes watching an image are practically fixed upon the image.
With a display method using the sub-field method of the prior art, however, a problem exists with dynamic images in that quality of the picture deteriorates due to the appearance of noise in the form of pseudo contours (i.e., "pseudo contours in dynamic images") unique to the dynamic images. This is described, for example, in "New Category Contour Noise Observed in Pulse-Width-Modulation Moving Images", ITEJ Technical Report by The of Institute of Television Engineers of Japan (Vol. 19, No. 2, IDY95-21, p. 61-66). People watching dynamic images on a screen consciously perceive moving objects that are in motion in the screen. In the sub-field method, a luminous level of any particular spot (picture element) of an image being caught by human eyes is in proportion to a normal sum of the illuminated time or the number of pulses within the elapsed time of one field, if it is of the still images. In case of dynamic images, however, a luminous level of a particular spot ("picture element") of the image is for human eyes in proportion to the sum of the illuminated time or the number of pulses which occur within a locus of the moving image because the image in that spot moves before the luminous level completely finishes at the spot. That is, an addition of the illuminated time or the number of pulses is made through a plurality of the picture elements rather than a single picture element. Therefore the quality of pictures deteriorates, as the eyes do not perceive the luminous level of each picture element in the dynamic images as their normal luminous level. This deterioration of picture quality is conspicuously perceivable in images in which the luminous level gradually varies among adjoining picture elements such as human faces and the skin, and pseudo contour patterns similar to contour lines appear. This phenomenon is now described using the figures.
FIG. 31 shows a condition in which four adjoining picture elements, "a", "b", "c" and "d" illuminate along with a lapse of time (axis of abscissa). In this instance, the picture elements "a" and "b" illuminate in the sub-fields 1, 2, 3, 4, 5, 6 and 7, but do not illuminate in the sub-field 8. On the other hand, the picture elements "c" and "d" do not illuminate in the sub-fields 1, 2, 3, 4, 5, 6 and 7, but they do illuminate in the sub-field 8. This means that the luminous level of the picture elements "a" and "b" is "127" and the luminous level of the picture elements "c" and "d" is "128" in FIG. 31, giving a typical example of two groups of picture elements each having the luminous levels of "127" and "128" adjoining each other with only one luminous level difference.
If the image stands still and the user's eyes stay fixed, the user watches the luminescence of all sub-fields along an arrow marked "fixed eyes 127" in FIG. 31, and correctly integrates the illuminated time or the number of pulses so as to perceive a luminous level of the luminous level "127" at the picture element having the luminous level of "127" in a screen. Likewise, the user watches the luminescence of all sub-fields along an arrow marked "fixed eyes 128" and perceives a luminous level of the luminous level "128" at the picture element having the luminous levels of "128" in the screen.
On the other hand with the dynamic images, however, a disorder occurs in luminous gradations in an image formed on the retina, since the eyes follow the moving image which causes a deviation in position of the picture elements in relation to the corresponding sub-fields as the time elapses.
Consider, as an instance, that an image moves a distance of three picture elements during a period of one field. That is, a particular image on the screen moves from a spot of the picture element "a" to a spot of the picture element "d" within the lapse of time (duration) for one field. In this situation, the human eyes gaze at the picture element "a" at a time when the sub-field 1 is illuminating, then follow the moving image in response to a speed of the image, and move on to the picture element "d" with the anticipation of the move after the duration of one field. This move is shown by a dotted line toward the lower right in FIG. 31. The eyes move from an upper left part to a lower right part in FIG. 31. Consequently, the eyes perceive a brightness of "255" in the luminous level (which equals (1+2+4+8+16+32+64)+128), as they observe all of the sub-fields 1 through 7 of the picture elements "a" and "b" both of which have a luminous level of "127", and the sub-field 8 of the picture elements "c" and "d" which have a luminous level of "128".
Conversely, the eyes may perceive a luminous level of "0", in the luminous level, because they catch the sub-fields while none of them is illuminating when the eyes move from the picture element "d" to the picture element "a", or from a lower left part to an upper right part of FIG. 31. This phenomenon, in which human eyes watching a dynamic image perceive an unintended luminous level when they follow a movement of the image, becomes more conspicuous when the eyes fail to recognize the luminescence of sub-fields having especially large weight ("luminous level").
As described, the half tones displaying method of the prior art has a problem in that it may cause the user to perceive an unnaturalness as if there is a difference in luminous level between picture elements that in fact have an imperceptible difference, when watching the screen by following the image in motion.